[Paper Review] A 96 GeV Higgs Boson in the 2HDM plus Singlet

S. Heinemeyer, C. Li+3 more|arXiv (Cornell University)|Jan 1, 2021

Particle physics theoretical and experimental studies4 citations

TL;DR

The paper proposes a 96 GeV Higgs boson within the 2HDM+Singlet framework (either 2HDMS or N2HDM) to simultaneously explain a 3σ diphoton excess at CMS and a 2σ b¯b excess at LEP. It demonstrates that the lightest CP-even Higgs in both models can fit both excesses while satisfying all LHC, Tevatron, LEP, and theoretical constraints. Precision measurements at the ILC250 could not distinguish between the two models due to identical coupling predictions for the 96 GeV state.

ABSTRACT

We discuss a $\sim 3\,\sigma$ signal (local) in the light Higgs-boson search in the diphoton decay mode at $\sim 96$ GeV as reported by CMS, together with a $\sim 2\,\sigma$ excess (local) in the $b \bar b$ final state at LEP in the same mass range. We interpret this possible signal as a Higgs boson in the 2 Higgs Doublet Model type II with an additional Higgs singlet, which can be either complex (2HDMS) or real (N2HDM). We find that the lightest CP-even Higgs boson of the two models can equally yield a perfect fit to both excesses simultaneously, while the second lightest state is in full agreement with the Higgs-boson measurements at $125$ GeV, and the full Higgs-boson sector is in agreement with all Higgs exclusion bounds from LEP, the Tevatron and the LHC as well as other theoretical and experimental constraints. We derive bounds on the 2HDMS and N2HDM Higgs sectors from a fit to both excesses and describe how this signal can be further analyzed at future $e^+e^-$ colliders, such as the ILC. We analyze in detail the anticipated precision of the coupling measurements of the $96$ GeV Higgs boson at the ILC. We find that these Higgs-boson measurements at the LHC and the ILC cannot distinguish between the two Higgs-sector realizations.

Motivation & Objective

• . The paper aims to interpret a 96 GeV diphoton excess in CMS and a 96 GeV b¯b excess in LEP as a single light Higgs boson in an extended Higgs sector.
• . It investigates whether the 2HDM type II with an additional real or complex singlet (N2HDM or 2HDMS) can simultaneously explain both excesses.
• . The study evaluates the compatibility of the full Higgs sector with all experimental constraints from LEP, Tevatron, and the LHC.
• . It assesses the potential of future e+e− colliders, particularly the ILC250, to probe the couplings of the 96 GeV Higgs boson.
• . The objective includes determining whether the 2HDMS and N2HDM models can be distinguished via precision measurements at future colliders.

Proposed method

• . The 2HDMS and N2HDM models are defined with distinct symmetries: Z3 for 2HDMS and Z′2 for N2HDM, both preserving tree-level FCNCs.
• . The Higgs potential is constructed with two doublets and a singlet, with vacuum expectation values (vevs) for all scalar fields.
• . Experimental constraints from LEP, Tevatron, and LHC Higgs exclusion bounds are applied, including precision measurements at 125 GeV.
• . The signal excesses are modeled using event counts and significance levels: 3σ local in CMS diphoton channel and 2.3σ local in LEP b¯b channel.
• . The ILC250 sensitivity is evaluated via signal-to-background ratios (S/B), cross-section calculations, and uncertainty propagation for coupling measurements.
• . Coupling uncertainties are derived using statistical error propagation: ∆NS/NS ∝ √(NT) and ∆NB ∝ ϵsyst,B·NB, with signal-to-background ratios used to estimate relative precision.

Experimental results

Research questions

• RQ1 . Can the 2HDM+Singlet framework (2HDMS or N2HDM) simultaneously explain the 96 GeV diphoton excess in CMS and the b¯b excess in LEP?
• RQ2 . Are the predictions for the 96 GeV Higgs boson's couplings identical in 2HDMS and N2HDM, such that they cannot be distinguished at future colliders?
• RQ3 . What are the bounds on the Higgs sector parameters in 2HDMS and N2HDM that are consistent with both excesses and all other constraints?
• RQ4 . How precisely can the couplings of the 96 GeV Higgs boson be measured at the ILC250?
• RQ5 . What role do loop corrections and background estimation uncertainties play in the precision of coupling measurements at future e+e− colliders?

Key findings

• . The lightest CP-even Higgs boson in both 2HDMS and N2HDM can simultaneously fit the 3σ CMS diphoton excess and the 2.3σ LEP b¯b excess with high consistency.
• . The second-lightest Higgs state in both models is fully compatible with the 125 GeV Higgs boson measurements at the LHC.
• . All Higgs sectors in both models satisfy all existing Higgs exclusion bounds from LEP, Tevatron, and the LHC.
• . The 96 GeV Higgs boson's couplings to fermions and gauge bosons are predicted with comparable precision in both models at the ILC250.
• . The signal-to-background ratio (S/B) for the 96 GeV Higgs in ILC250 is estimated with D = fh/fφ ≈ 3 as a conservative value, leading to measurable coupling precisions.
• . The ILC250 cannot distinguish between 2HDMS and N2HDM for the 96 GeV Higgs due to identical coupling predictions, even with high-precision measurements.